Over one hundred cases of legionnaires' disease have been linked to ships, and ten cases are known to have died. Most of the cases were associated with cruise ships, but a variety of other vessels were also linked to cases. Few vessels were investigated microbiologically, and the cases associated with ferries were exposed to other sources of infection. Cases appear to be less common among crew members than among passengers. To prevent further cases, ship owners, operators, and captains need to be diligent in maintaining the water and air conditioning systems of their vessels. Whirlpool spas need particular care. Ship-associated cases of non-pneumonic legionellosis appear to be rare.